Ever reducing design rule has become more strict to meet the requirements to increase the scale of integration and speed of the LSI. Reduction in wiring pitch has led to reduction in the width of wiring and distance between the wirings. However, reduction in thickness of wiring has not been largely achieved in comparison with the reduction in wiring pitch in order to prevent an increase in resistance of wiring.
Accordingly, the aspect ratio of the wiring height to the wiring width or distance between wirings tends to increase. Embedding of metal has become more difficult in particular when trench wiring is used. Similarly, the aspect ratio of via for connecting with an active area or underlying wiring has also increased, so that embedding of metal has become more difficult.
Efforts to use materials having a low resistance have been actively made to increase the speed of devices. Researches have been carried out to use copper-based wiring materials in lieu of conventional aluminum-based wiring materials.